1. Field of the Invention
The present invention relates to an X-ray detector for converting X-rays into an electric signal and, more particularly, to an X-ray detector, incorporated in an X-ray computed tomography scanner, for detecting X-rays.
2. Description of the Related Art
In an X-ray computed tomography scanner of the third or fourth generation, an X-ray detector in which a plurality of X-ray detecting elements are arranged in an arcuated shape is used and X-rays passing through a region of interest are detected by the X-ray detecting elements. Recently, such a detecting element is manufactured by combining a scintillator element and a photo-detecting element as disclosed in U.S. Pat. No. 4,234,792. X-rays incident on the scintillator element are converted into light rays by the scintillator element, and the converted light rays are converted into an electric signal by the photo-detecting element fixed to the scintillator element.
An X-ray detector for an X-ray computed tomography scanner comprises a large number of X-ray detecting elements each consisting of a photo-detecting semiconductor structure and a scintillator element. A large number of photo-detecting semiconductor structures are arranged in an array on a single chip cut out from a wafer. A large number of scintillator elements are similarly arranged in an array, and this array of scintillator elements is mounted on the single chip so that each scintillator element is aligned with a corresponding one of the photo-detecting structures. In such an X-ray detector, a width of each photo-detecting structure in an array direction of the structures is set substantially equal to or slightly smaller than that of each scintillator element in the same array direction. Therefore, if a small alignment error is produced due to a manufacturing error or an adverse affect of thermal expansion during manufacture, a semiconductor region between the adjacent photo-detecting structures is brought into contact with the lower surface of each scintillator element. In this case, light rays generated in each scintillator element are incident on the semiconductor region and are converted into electric charge in the semiconductor region. These electric charges may be disadvantageously diffused and supplied to an adjacent photo-detecting structure as noise. As a result, a signal component generated in a scintillator element not corresponding to a photo-detecting structure into which the electric charges are flowing is detected by a photo-detecting structure, and crosstalk is generated between adjacent X-ray detecting elements each consisting of a photo-detecting structure and a scintillator element.
The X-ray detector for an X-ray computed tomography scanner is normally constituted by 512 X-ray detecting elements. In general, the detector is manufactured by assembling 32 units each consisting of 16 X-ray detecting elements. As described above, a chip of each unit on which the photo-detecting structure array is formed is cut out from a wafer. Therefore, a signal current, i.e., a dark current may be generated in a photo-detecting structure adjacent to a cutting side of each chip without incidence of light rays. This is because in the cutout chip, when defects are produced in a semiconductor crystal structure of the cutting side and an operation voltage is applied to a photo-detecting structure, a dark current is generated in this region and flowed into a photo-detecting structure adjacent to the cutting side as noise.